The present invention concerns a method for the isolation of cobalt and/or manganese in the form of precipitated, easily filtrable cobalt and/or manganese oxalates from solutions of cobalt and/or manganese compounds, in addition, in some cases, to other heavy metal compounds and organic and/or inorganic substances, in aliphatic carboxylic acids or water or in mixtures thereof.
The present method is especially suitable for the isolation of cobalt and/or manganese salts from acetic acid oxidation mother liquors in which they have served as catalysts in the oxidation of alkyl aromatics with atmospheric oxygen in acetic acid solution for the preparation of aromatic carboxylic acids. Such oxidation processes are of great economic inportance. This is especially true of the production of terephthalic acid by the oxidation of p-xylene. Similar methods, however, are used in preparing other mononuclear or polynuclear aromatic mono-, di- or polycarboxylic acids, such as for example chlorobenzoic or nitrobenzoic acid, isophthalic acid, naphthalic dicarboxylic acid, trimellitic acid etc.
An important factor in the economy of such processes is the recovery of the catalyst. In most of the methods described for the recovery of catalyst from oxidation mother liquors, such as the one described, for example, in DE-OS No. 2,131,470, DE-OS No. 2,419,323 or DE-OS No. 2,260,491, the catalysts are isolated as carbonates. For this purpose, however, the acetic acid solutions must first be concentrated and the residues extracted with water. The extraction of this often tarry, and occasionally even 2-phase residue, is problematic and not always sufficiently effective. An incomplete extraction of the cobalt, however, signifies not only losses of the relatively expensive catalyst, but also makes it appreciably more difficult to get rid of the residual product which then still contains cobalt.
A method for the recovery of cobalt and/or manganese catalyst from acetic acid oxidation mother liquors is described in Japanese OS No. 97593/76. In this case the heavy metal catalyst in the oxidation mother liquor is precipitated in the form of a difficultly soluble oxalate by the addition of oxalic acid, and isolated by a solid-from-liquid separating method. This process offers the possibility of separating other heavy metal oxalates precipitated together with the cobalt and/or manganese oxalate by washing them out.
When the method of the Japanese patent application was attempted, the cobalt oxalate precipitated from acetic acid oxidation mother liquors or from acetic acid solutions of cobalt acetate proved to be virtually unfiltrable. Even when the precipitation conditions were varied as regards temperature, degree of aqueous dilution of the solution, cobalt concentration or method of adding the oxalic acid, easy filtrability could not be achieved. The addition of filter aids brought no improvement. In all cases the cobalt and/or manganese oxalate was obtained in a form in which it was virtually unfiltrable by means of filtration centrifuges, or suction or pressure filters.
Examination with the scanning electron microscope of the cobalt oxalate obtained in Prior-Art Example 1 revealed the reason for the extraordinary difficulty encountered in filtration: the crystallizate consisted of small, rounded primary particles with a diameter of only about 0.1 to 0.3 microns. It is thus apparent that the filtration of this extremely finely crystalline product with conventional filtration apparatus was incomplete and could be accomplished only in a very time-consuming manner, even using a disproportionately large filtration surface area. In the case of relatively great layer thicknesses, filtration was completely impossible, since then the small, rounded crystals were packed together into a very dense layer blocking the flow of liquid.